This invention relates to a method and a punch/die assembly for use in the production of heat exchanger fins as in, for example, air conditioners for rooms and motor vehicles and, in particular, to an assembly wherein an ironing punch is correctly maintained in a predetermined state during the ironing process.
In general, the cooling fins are constituted by a number of superposed square aluminium sheets each having a multiple number of flanged apertures in registration with similar flanged apertures in the overlying and underlying sheets, and a number of copper tubes passed through the respective flanged apertures. The flanges are usually turned outwardly to form reflaring around the marginal edges of the apertures for the purpose of maintaining a predetermined clearance between the adjacent apertures and at the same time for reinforcing purposes. Therefore, the flanges are required to have at least projecting and curling height by more than 1.8 mm.
Referring to FIG. 1 of the accompanying drawings, in forming a flanged aperture in an aluminum sheet 1, it has been the conventional practice to perforate in the first step an aperture 2 which has a diameter far smaller than that of a flanged aperture 4 to be ultimately formed, pressing the marginal edge portions upwardly by means of a punch 3 thereby forming the flanged aperture of the predetermined dimension. However, where the aperture 2 is formed in a small diameter in an attempt to increase the height of the ultimate flange, cracking often occurs to the marginal edge portions of the aperture when pressed by the punch. Therefore, without a preliminary treatment or machining, it has been difficult to form a flange which has a height greater than 1.8 mm.
The pre-machining usually includes pressing of the coiled aluminum material 1 by a punch to form a bonnet-like recess 5 of a diameter far larger than that of the intended flanged aperture, as shown in FIG. 2(A), and further pressing of the recessed portion 5 by another punch 6 to reduce its diameter while increasing its height as shown particularly in FIG. 2(B). These operations are repeated to obtain a number of flanged apertures 4 of the predetermined diameter and height as shown in FIG. 1. This method is generally referred to as "drawing" and is capable of forming a flange of a relatively great height by the gradual or progressive stretching of the coiled aluminum material 1. However, the just-mentioned method has inherent drawbacks in that the circumferential wall of the flanged aperture 4 bears concentric hammered marks as a result of the repeated punching operation and wrinkles appear at both ends of the coiled aluminum material 1 to cause warping or distortion to the fins as a whole.
Concerning the above-mentioned problems, the inventor of the present application developed a forming method (refer to U.S. patent application Ser. No. 604,306 filed on Aug. 13, 1975 now U.S. Pat. No. 4,055,067) which comprises: the first process of perforating a small aperture at a predetermined position of coiled aluminum material, without the afore-mentioned pretreatment, while simultaneously forming a perpendicularly projecting cylinder of a diameter smaller than that of the flanged aperture to be ultimately formed, and the second process of burring and ironing the perpendicularly projecting cylinder into a predetermined dimension with use of a punch of a predetermined size.
In FIG. 3(A), a small aperture 11 is perforated in the coiled aluminum material in the form of a sheet 1 by menas of a punch 7. As a punch 9 is urged into a die 8, a relatively short cylinder 10 is formed around the small aperture 11 in the coiled aluminum material which is held against the lower surface of the die 8, as shown in FIG. 3(B). The coiled aluminum material 1, placed between another die 12 and a punch 13 which is designed to have a size conforming with the die 12 is pressed into the die 12 to form the perpendicular cylinder 10 into a flange 14 of a predetermined height as illustrated in FIGS. 3(C), 3(D) and 3(E).
In the actual operation, the afore-mentioned first and second process are carried out with use of a set of dies 8 and 12 and a set of punches 9 and 13 which are supported on a punch/die assembly 40, as shown in FIGS. 4 and 5. The punch and die assembly 40 comprises an upper tool holder 16 and a lower tool holder 15 having integrally therewith a stripper plate 18 which is constantly urged upwardly by springs within a guide frame 17. The upper and lower tool holders 16 and 15 of the punch and die assembly 40 are mounted on a suitable press machine such that the upper tool holder 16 is pressed downwardly against spring action to effect the afore-mentioned first and second punching operations. More particularly, the upper tool holder 16 has the die 8 for the first punching operation and the die 12 for the second punching operation mounted thereon, the dies 8 and 12 being aligned in the direction of advancement of the workpiece 1 (from left to right as seen in FIG. 5) in one or a plural number of sets. The coiled aluminum material or workpiece 1 undergoes the first and second punching operations as it is moved intermittently or incrementally by means of a hitch-feeding mechanism 19 which is provided separately from the punch and die assembly 40. The just-mentioned dies 8 and 12 and the punches 9 and 13 are preferably provided in a plural number and arrayed, respectively, in the lateral direction (in the direction perpendicular to the workpiece feeding direction) to simultaneously form a plural number of laterally aligned flanged apertures 14 in relation with the intermittent movement of the workpiece 1. The hitch-feeding mechanism 19 in the coiled aluminum material is driven in timed relation with the reciprocating movement of the upper tool holder 16.
According to the method as shown in FIGS. 3(A) - (E), a small perpendicularly projecting cylinder 10 is formed by a punch 7 in the first process at a desired position on the coiled aluminum material 1, and in the second process, the perpendicularly projecting cylinder 10 is further pressed out by a punch 13. In this instance, the upper circumferential portions of the perpendicularly projecting cylinder 10 is deformed by the punch 13, during the process as shown in FIGS. 3(C) - 3(D), but with only a reduced tensioning stress, so that a perpendicular flange 14, which has a height of 2.8 mm in the particular embodiment shown, may be formed without difficulty and at the same time without causing cracking or other problems. However, there poses a problem. That is, in the forming steps shown in FIGS. 3(C), (D), and (E), a spacing between the die 12 and the punch 13 must be invariably fixed in every area thereof so that the marginal edge portions of the perpendicularly projecting cylinder 10 can be formed to have equal height, namely the center of the punch 13 must correctly be consistent with the center of the die 12. If these are disposed eccentrically towards one side, the peripheral walls of the perpendicularly projecting cylinder 10 are not formed to have equal height due to the change in wall thickness, but rather formed into a diagonally cut edge which inclines towards one side as shown in FIG. 6. For example, there occurs a difference in altitude of 0.2 mm with respect to 5/1000 mm eccentricity and therefore, this adjustment must be made precisely accurate.